Leishmania resides in the alimentary tract of vector sandflies, in a medium comprising the food of the fly mixed with digestive enzymes. We shall determine whether variations in the composition of the food influence the course of development of the pathogen and whether such factor may affect the natural force of transmission. One series of experiments will correlate the presence of various naturally occurring food plants and honeydew, found in the Jordan Valley, with the course of L. major infection in P. papatasi. First, these foods will be provided to laboratory-reared infected flies and the effect of each diet characterized by estimating parasite density, mortality and morphological abberations, in stained preparations of gut-contents. The criteria deduced from these experiments will be used to screen putative field diets for their effect on natural infections in vector flies. Then, we shall evaluate how frequently each of these foods are ingested in nature, by examining the guts of wild-caught flies. The prevalence of Leishmania will be correlated with the presence of particular food sources. A companion series of experiments will investigate whether blood of different animals, endemic to the Jordan Valley, affects the course of infection in sandflies. Thus, we shall determine whether leishmanial infection is diminished or eliminated after infected flies feed on particular naturally occurring animals. Because midgut proteolysis in vector sandflies varies between different kinds of hosts and the major glycoconjugates of the Leishmania influences these digestive enzymes, we shall compare the trypsin-like enzymes secreted in sandflies after feeding on this array of hosts. The proposed analysis of the diet of the vector as a factor that affects vector competence and the force of leishmanial transmission constitutes a novel approach to the epidemiology of the leishmaniases. We anticipate that these results will contribute to a basic understanding of vector- pathogen interactions and may provide methods useful in the control of these infections.